Showing papers on "Flow separation published in 2005"

Aspects of low- and high-frequency actuation for aerodynamic flow control

Abstract: Control approaches for separated flows over aerodynamic (or bluff) bodies in which the separated flow domain scales with the characteristic length of the body are distinguished by the frequency band of the actuation input. In an approach that relies on the narrowband receptivity of the separating shear layer that is coupled to the wake (shedding) instability and scales with the characteristic advection time over the separated domain, aerodynamic performance is partially restored by a Coanda-like deflection of the forced separating shear layer toward the surface. Because the instability of the unforced shear layer may already be driven by global vortex shedding, the advection of the vortices of the forced (or controlled) layer along the surface and their ultimate shedding into the near wake can couple to wake instabilities and, therefore, may result in unsteady aerodynamic forces in the controlled flow. A different control strategy that emphasizes full or partial suppression of separation by fluidic modification of the apparent aerodynamic shape of the surface relies on controlled interaction between the actuator and the crossflow on a scale that is at least an order of magnitude smaller than the relevant global length scales.

Numerical Simulations of Plasma Based Flow Control Applications

Abstract: A mathematical model was developed to simulate flow control applications using plasma actuators. The effects of the plasma actuators on the external flow are incorporated into Navier Stokes computations as a body force vector. In order to compute this body force vector, the model solves two additional equations: one for the electric field due to the applied AC voltage at the electrodes and the other for the charge density representing the ionized air. The model is calibrated against an experiment having plasma-driven flow in a quiescent environment and is then applied to simulate a low pressure turbine flow with large flow separation. The effects of the plasma actuator on control of flow separation are demonstrated numerically.

Aerodynamic transfer of energy to the vocal folds

Abstract: The aerodynamic transfer of energy from glottal airflow to vocal fold tissue during phonation was explored using complementary synthetic and numerical vocal fold models. The synthetic model was fabricated using a flexible polyurethane rubber compound. The model size, shape, and material properties were generally similar to corresponding human vocal fold characteristics. Regular, self-sustained oscillations were achieved at a frequency of approximately 120 Hz. The onset pressure was approximately 1.2 kPa. A corresponding two-dimensional finite element model was developed using geometry definitions and material properties based on the synthetic model. The finite element model upstream and downstream pressure boundary conditions were based on experimental values acquired using the synthetic model. An analysis of the fully coupled fluid and solid numerical domains included flow separation and unsteady effects. The numerical results provided detailed flow data that was used to investigate aerodynamic energy transfer mechanisms. The results support the hypothesis that a cyclic variation of the orifice profile from a convergent to a divergent shape leads to a temporal asymmetry in the average wall pressure, which is the key factor for the achievement of self-sustained vocal fold oscillations. me rica.

Measurements of the flow over a low-aspect-ratio cylinder mounted on a ground plane

Abstract: The flow over a finite-height cylinder of aspect ratio 1, with one end mounted on a ground plane and the other end free, has been studied by means of surface flow visualisation, particle image velocimetry (PIV) and surface pressure measurements. The diameter-based Reynolds number was 200,000. The mean flow topology has been identified in three areas: the horseshoe vortex system, the separated flow over the free-end and the wake region. Evidence is shown for the existence of a three-horseshoe vortex system, while the mean flow over the free-end consists of an arch vortex with its bases on the forward half of the free-end. There are two tip vortices coming off the free-end. The wake region is found to be highly unsteady, with considerable variation from the mean flow.

Passive micromixers for applications in the microreactor and μTAS fields

Abstract: An overview is given of current developments in micromixing technology, where the emphasis is on liquid mixing in passive micromixers. The mixers presented are differentiated by the hydrodynamic principle employed, and four important principles are discussed in some detail: hydrodynamic focusing, flow separation, chaotic advection, and split-and-recombine flows. It is shown that these principles offer excellent mixing performance in various dynamical regimes. Hydrodynamic focusing is a concept working very much independently of the Reynolds number of the flow. Flow separation offers rich dynamical behavior over a Reynolds number scale of several hundred, with superior performance compared to purely diffusive mixing already found at low Reynolds numbers. For chaotic advection, different implementations tailor-made for low and comparatively high Reynolds numbers exist, both leading to an exponential increase of the interface between two fluids. Split-and-recombine flows can only be realized in a close-to-ideal form in the low Reynolds number regime. Corresponding mixers can be equipped with comparatively wide channels, enabling a favorable ratio of throughput to pressure drop. The overview given in this article should enable a potential user of micromixing technology to select the most favorable concept for the application envisaged, especially in the field of chemical process technology

Bladerow interactions, transition, and high-lift aerofoils in low-pressure turbines

Abstract: ▪ Abstract The flow in turbomachines is unsteady due to the relative motion of the rows of blades. In the low-pressure turbine, the wakes from the upstream bladerows provide the dominant source of unsteadiness. Because much of the blade-surface boundary-layer flow is laminar, one of the most important consequences of this unsteadiness is the interaction of the wakes with the suction-side boundary layer of a downstream blade. This is important because the blade suction–side boundary layers are responsible for most of the loss of efficiency and because the combined effects of random (wake turbulence) and periodic disturbances (wake velocity defect and pressure fields) cause the otherwise laminar boundary layer to undergo transition and eventually become turbulent. This article summarizes the process of wake-induced boundary-layer transition in low-pressure turbines and the loss generation processes that result. Particular emphasis is placed on how the effects of wakes may be exploited to control loss genera...

Large-eddy simulation of low frequency oscillations of the Dean vortices in turbulent pipe bend flows

Abstract: Large-eddy simulations are performed to investigate turbulent flows through 90° pipe bends that feature unsteady flow separation, unstable shear layers, and an oscillation of the Dean vortices Single bends with curvature radii of one- and three-pipe diameters are considered at the Reynolds number range 5000–27 000 The numerically computed distributions of the time-averaged velocities, Reynolds stress components, and power spectra of the velocities are validated by comparison with particle image velocimetry measurements The power spectra of the overall forces onto the pipe walls are determined The spectra exhibit a distinct peak in the high frequency range that is ascribed to vortex shedding at the inner side of the bends and shear layer instability At the largest Reynolds number the spectra also exhibit an oscillation at a frequency much lower than that commonly observed at vortex shedding from separation It turns out that the associated flow pattern is similar to the swirl switching phenomenon earl

Flow past a cylinder: shear layer instability and drag crisis

Abstract: Flow past a circular cylinder for Re=100 to 107 is studied numerically by solving the unsteady incompressible two-dimensional Navier–Stokes equations via a stabilized finite element formulation. It is well known that beyond Re ∼ 200 the flow develops significant three-dimensional features. Therefore, two-dimensional computations are expected to fall well short of predicting the flow accurately at high Re. It is fairly well accepted that the shear layer instability is primarily a two-dimensional phenomenon. The frequency of the shear layer vortices, from the present computations, agree quite well with the Re0.67 variation observed by other researchers from experimental measurements. The main objective of this paper is to investigate a possible relationship between the drag crisis (sudden loss of drag at Re ∼ 2 × 105) and the instability of the separated shear layer. As Re is increased the transition point of shear layer, beyond which it is unstable, moves upstream. At the critical Reynolds number the transition point is located very close to the point of flow separation. As a result, the shear layer eddies cause mixing of the flow in the boundary layer. This energizes the boundary layer and leads to its reattachment. The delay in flow separation is associated with narrowing of wake, increase in Reynolds shear stress near the shoulder of the cylinder and a significant reduction in the drag and base suction coefficients. The spatial and temporal power spectra for the kinetic energy of the Re=106 flow are computed. As in two-dimensional isotropic turbulence, E(k) varies as k−5/3 for wavenumbers higher than energy injection scale and as k−3 for lower wavenumbers. The present computations suggest that the shear layer vortices play a major role in the transition of boundary layer from laminar to turbulent state. Copyright © 2004 John Wiley & Sons, Ltd.

A New Class of Synthetic Jet Actuators—Part II: Application to Flow Separation Control

Abstract: We present the application of the new synthetic jet actuator (SJA) to flow separation control over a NACA 0015 wing. The actuator is compact enough to fit in the interior of the wing that has a chord of 0.375 m. The wing was tested in the Texas A&M University Aerospace Engineering 3 ftX4 ft wind tunnel. An experimental investigation into the effects of the synthetic jet actuator on the performance of the wing is described. Emphasis is placed on the capabilities of the actuator to control the separation of the flow over the wing at high angles of attack. The results include force balance measurements, on surface and off surface flow visualization, surface pressure measurements, and wake surveys. All of the reported tests were performed at a free-stream velocity of 35 m/s, corresponding to a Reynolds number of 8.96×10 5 . The angle of attack was varied from -2.0 deg to 29.0 deg.

Large-scale turbulent flow around a cylinder in counterflow superfluid 4 He (He ( II ))

Abstract: The detailed nature of fluid flow over a cylinder is one of the fundamental topics in classical fluid dynamics as it demonstrates flow separation and vortex shedding1. In superfluid helium, either He (II) or the B phase of3He, an important question has been to what extent these quantum fluids show classical fluid turbulent states2,3,4. Although the existence of turbulent structures can be inferred using precise instrumentation5,6, direct visualization of the flow field can provide unequivocal evidence of these phenomena. Here we show the existence of large turbulent structures in He (II) counterflow across a cylinder as obtained by the particle image velocimetry technique. Compared with classical fluid flow, the particle motion driven by He (II) counterflow shows macroscopic eddies downstream of the cylinder but also similar structures are observed in front of the cylinder, behaviour not seen in classical fluids. As Landau’s two-fluid model7 for He (II) describes counterflow as the relative motion of the superfluid and normal fluid components, the current results indicate that both components may be undergoing a kind of flow separation as they pass over the cylinder.

Reynolds number effects on the behavior of a non-buoyant round jet

Abstract: The behavior of a non-buoyant circular water jet discharged from a contraction nozzle was experimentally investigated. In this experiment, the Reynolds number of the jet, based on the mean velocity results obtained by particle image velocimetry (PIV), ranged from 177 to 5,142. From the experimental results, we found that the cross-sectional profile of the axial velocity for a laminar flow near the nozzle did not show a top-hat distribution, whereas the profiles with Reynolds number higher than 437 were almost top-hat. The length of the zone of flow establishment (ZFE) was found to decrease with increasing Reynolds number. The measured centerline velocity decayed more rapidly and, consequently, approached the theoretical equation earlier near the nozzle as the Reynolds number increased. The decay constant for the centerline velocity of the turbulent cases was relatively lower than that discovered in theory. It is assumed that this probably resulted from the use of the contraction nozzle. Verifying the similarity of the lateral velocity profiles demonstrated that the Gaussian curve was properly approximated only for the turbulent jets and not for the laminar or transitional flows. The jet half width seldom grew for the laminar or transitional flows, whereas it grew with increasing axial distance for the turbulent flows. The spreading rates for the turbulent flows gradually decreased with increasing Reynolds number. The normalized turbulence intensity along the jet centerline increased more rapidly with the axial distance as the Reynolds number increased, and tended to the constant values proposed by previous investigators. The Reynolds shear stress levels were also found to increase as the Reynolds number increased for the turbulent jets.

Laminar transitional and turbulent flow of yield stress fluid in a pipe

Abstract: This paper presents an experimental study of the laminar, transitional and turbulent flows in a cylindrical pipe facility (5.5 m length and 30 mm inner diameter). Three fluids are used: a yield stress fluid (aqueous solution of 0.2% Carbopol), a shear thinning fluid (aqueous solution of 2% CMC) without yield stress and a Newtonian fluid (glucose syrup) as a reference fluid. Detailed rheological properties (simple shear viscosity and first normal stress difference) are presented. The flow is monitored using pressure and (laser Doppler) axial velocity measurements. The critical Reynolds numbers from which the experimental results depart from the laminar solution are determined and compared with phenomenological criteria. The results show that the yield stress contribute to stabilize the flow. Concerning the transition for a yield stress fluid it has been observed an increase of the root mean square ( rms ) of the axial velocity outside a region around the axis while it remains at a laminar level inside this region. Then, with increasing the Reynolds number, the fluctuations increase in the whole section because of the apparition of turbulent spots. The time trace of the turbulent spots are presented and compared for the different fluids. Finally, a description of the turbulent flow is presented and shows that the rms axial velocity profile for the Newtonian and non-Newtonian fluids are similar except in the vicinity of the wall where the turbulence intensity is larger for the non-Newtonian fluids.

Direct numerical simulation of polymer-induced drag reduction in turbulent boundary layer flow

Abstract: We describe a method for direct numerical simulation of polymer-induced friction drag reduction in turbulent boundary layers. The effect of the polymer additives that induce spatial variations of skin-friction drag is included in the momentum equation through a continuum constitutive model for the viscoelastic stress, which is based on the evolution of a parameter describing the fluid microstructure. We demonstrate that the turbulence structure and polymer microstructure evolve asynchronously as one moves in the streamwise direction. We observe an initial development length, which is followed by a quasisteady region where variations in drag reduction are weak. High drag reduction behavior can be present at short downstream distances from the inflow plane.

Characterization of Unsteady Flow Structures Around Tandem Cylinders for Component Interaction Studies in Airframe Noise

Abstract: A joint computational and experimental study has been performed at NASA Langley Research Center to investigate the unsteady flow generated by the components of an aircraft landing gear system. Because the flow field surrounding a full landing gear is so complex, the study was conducted on a simplified geometry consisting of two cylinders in tandem arrangement to isolate and characterize the pertinent flow phenomena. This paper focuses on the experimental effort where surface pressures, 2-D Particle Image Velocimetry, and hot-wire anemometry were used to document the flow interaction around the two cylinders at a Reynolds Number of 1.66 x 10(exp 5), based on cylinder diameter, and cylinder spacing-todiameter ratios, L/D, of 1.435 and 3.70. Transition strips were applied to the forward cylinder to produce a turbulent boundary layer upstream of the flow separation. For these flow conditions and L/D ratios, surface pressures on both the forward and rear cylinders show the effects of L/D on flow symmetry, base pressure, and the location of flow separation and attachment. Mean velocities and instantaneous vorticity obtained from the PIV data are used to examine the flow structure between and aft of the cylinders. Shedding frequencies and spectra obtained using hot-wire anemometry are presented. These results are compared with unsteady, Reynolds-Averaged Navier-Stokes (URANS) computations for the same configuration in a companion paper by Khorrami, Choudhari, Jenkins, and McGinley (2005). The experimental dataset produced in this study provides information to better understand the mechanisms associated with component interaction noise, develop and validate time-accurate computer methods used to calculate the unsteady flow field, and assist in modeling of the radiated noise from landing gears.

Flow in a mechanical bileaflet heart valve at laminar and near-peak systole flow rates: CFD simulations and experiments

Abstract: Time-accurate, fully 3D numerical simulations and particle image velocity laboratory experiments are carried out for flow through a fully open bileaflet mechanical heart valve under steady (nonpulsatile) inflow conditions. Flows at two different Reynolds numbers, one in the laminar regime and the other turbulent (near-peak systole flow rate), are investigated. A direct numerical simulation is carried out for the laminar flow case while the turbulent flow is investigated with two different unsteady statistical turbulence modeling approaches, unsteady Reynolds-averaged Navier-Stokes (URANS) and detached-eddy simulation (DES) approach. For both the laminar and turbulent cases the computed mean velocity profiles are in good overall agreement with the measurements. For the turbulent simulations, however, the comparisons with the measurements demonstrate clearly the superiority of the DES approach and underscore its potential as a powerful modeling tool of cardiovascular flows at physiological conditions. The study reveals numerous previously unknown features of the flow.

Reynolds number effect on drag reduction in a microbubble-laden spatially developing turbulent boundary layer

Abstract: We have performed direct numerical simulations for a microbubble-laden spatially developing turbulent boundary layer (SDTBL) and compared the amount of skin friction reduction due to the presence of the bubbles for two Reynolds numbers: , created by the bubble concentration gradients. Thus, the volume fraction of bubbles that is responsible for the reduction of skin friction in a SDTBL at a given Reynolds number is not sufficient to produce the same amount of reduction in skin friction at higher Reynolds number.

Reynolds number effect on drag reduction in a microbubble-laden spatially-developing turbulent boundary layer

Abstract: We have performed direct numerical simulations for a microbubble-laden spatially developing turbulent boundary layer (SDTBL) and compared the amount of skin friction reduction due to the presence of the bubbles for two Reynolds numbers: $Re_{\\theta}{=}1430$ and $Re_{\\theta}{=}2900$. The results show that increasing the Reynolds number decreases the percentage of drag reduction. Increasing $Re_{\\theta}$ ‘squeezes’ the quasi-streamwise vortical structures toward the wall, whereas the microbubbles ‘push them away’ from the wall. The net result of these two opposing effects determines the amount of skin friction reduction by the microbubbles. The displacement of the vortical structures by the microbubbles is a result of the local positive velocity divergence, $\\bm\ abla {\\bm\\cdot} {\\bm U}$, created by the bubble concentration gradients. Thus, the volume fraction of bubbles that is responsible for the reduction of skin friction in a SDTBL at a given Reynolds number is not sufficient to produce the same amount of reduction in skin friction at higher Reynolds number.

Heat transfer and aerodynamics of turbine blade tips in a linear cascade

Abstract: Local measurements of the heat transfer coefficient and pressure coefficient were conducted on the tip and near tip region of a generic turbine blade in a five-blade linear cascade. Two tip clearance gaps were used: 1.6% and 2.8% chord. Data was obtained at a Reynolds number of 2.3 X 10 5 based on exit velocity and chord. Three different tip geometries were investigated: A flat (plain) tip, a suction-side squealer, and a cavity squealer. The experiments reveal that the flow through the plain gap is dominated by flow separation at the pressure-side edge and that the highest levels of heat transfer are located where the flow reattaches on the tip surface. High heat transfer is also measured at locations where the tip-leakage vortex has impinged onto the suction surface of the aerofoil. The experiments are supported by flow visualization computed using the CFX CFD code which has provided insight into the fluid dynamics within the gap. The suction.vide and cavity squealers are shown to reduce the heat transfer in the gap but high levels of heat transfer are associated with locations of impingement, identified using the flow visualization and aerodynamic data. Film cooling is introduced on the plain tip at locations near the pressure-side edge within the separated region and a net heat flux reduction analysis is used to quantify the performance of the successful cooling design.

Laminar Flow and Heat Transfer in a Periodic Serpentine Channel

Abstract: The fully developed laminar flow and heat transfer behavior in periodic serpentine circular-section channels has been studied using computational fluid dynamics (CFD). The serpentine elements are characterised by their wavelength (2 L), channel diameter (d), and radius of curvature of bends (Rc) and results are reported for (3 ∼ 200, the flow becomes unsteady. Constant wall heat flux (H2) and constant wall temperature (T) boundary conditions have been examined for a range of fluid Prandtl number (0.7 < Pr < 100). The formation of Dean vortices produces significant heat transfer enhancement relative to flow in a straight pipe, with the effect being greater at higher values of Pr. Pressure drop is also increased but to a lesser extent. The alignment of the flow with the vorticity in these structures allows significant mixing of the fluid without creating large pressure-drop penalties. Dean vortices are also found to inhibit flow separation. These results suggest an effective method for enhancement of heat transfer in deep laminar flows.

Turbulence development in a non-equilibrium turbulent boundary layer with mild adverse pressure gradient

Abstract: High-resolution laser-Doppler anemometer measurements were acquired in a two-dimensional turbulent boundary layer over a ramp. The goals were to provide a detailed data set for an adverse pressure gradient boundary layer far from separation and to examine near-wall behaviour of the Reynolds stresses as compared to flat-plate boundary layers. The flow develops over a flat plate, reaching a momentum thickness Reynolds number of 3350 at an upstream reference location. The boundary layer is then subjected to a varying pressure gradient along the length of the ramp and partially redevelops on a downstream flat plate. Mean velocity measurements show a log law region in all velocity profiles, but the outer layer does not collapse in deficit coordinates indicating that the boundary layer is not in equilibrium. Measurements of non-dimensional stress ratios and quadrant analysis of the two-component data indicate relatively small changes to the turbulence structure. However, the streamwise normal stress has an extended outer layer plateau, and the shear stress and wall-normal stress have outer layer peaks. Near the wall, the streamwise normal stress and shear stress collapse with flat-plate data using standard scaling, but the wall normal stress is substantially larger than flat-plate cases.

Plasma Actuators for Landing Gear Noise Reduction

Abstract: A primary component of airframe noise on both takeoff and landing approach is due to the landing gear. The inherent bluff body characteristics of the landing gear give rise to large-scale flow separation that results in noise production through unsteady wake flow and large-scale vortex instability and deformation. In this paper the use of single dielectric barrier discharge plasma actuator technology for landing gear noise control is explored. Proof-of-concept experiments that use plasma actuators to create an effective "plasma fairing" which minimizes flow separation over the gear are presented. Nomenclature D = cylinder diameter D Re = Reynolds number based on cylinder diameter D St = Strouhal number based on cylinder diameter ∞ U = free stream velocity * b f = body force (per unit volume) vector φ = electric potential D λ = Debye length 0 e = electrical permittivity of free space

Numerical Investigation of Low-Aspect-Ratio Wings at Low Reynolds Numbers

Abstract: A numerical analysis is performed to study the flow around low-aspect-ratio (LAR) wings and more particularly the resulting lift-and-drag force. The research is focused on low-Reynolds-number aerodynamics, as LAR wings are crucial for the development of microair vehicles (MAVs). The flow around LAR wings is characterized by complex three-dimensional flow phenomena. These phenomena include wing-tip vortices, flow separation and reattachment, laminar to turbulent transition, and a mutual interaction among these phenomena. The flow is studied using a commercial computational fluid dynamics (CFD) program and a strip method. The CFD code is used to investigate the three-dimensional flow aerodynamics of rectangular LAR wings with an aspect ratio between 0.5 and 2 at a Reynolds number of 1 x 10 5 . Simulations on a flat plate and a reflex-type low-Reynolds-number profile (S5010), which is representative for a flying-wing MAV, are performed and compared. Experimental data is used for comparison and validation. The effects of flow separation and low Reynolds numbers are further investigated using a strip method. Two accurate formalized methods to predict lift and drag are derived. The first method is applicable to profiled wings with moderate low-Reynolds-number effects. The second method, which is based on the strip method, is more general and is also applicable to flat plates and wings exhibiting large regions of flow separation.

Observations of asymmetrical flow behaviour in transitional pipe flow of yield-stress and other shear-thinning liquids

Abstract: The purpose of this brief paper is to report mean velocity profile data for fully developed pipe flow of a wide range of shear-thinning liquids together with two Newtonian control liquids. Although most of the data reported are for the laminar–turbulent transition regime, data are also included for laminar and turbulent flow. The experimental data were obtained in unrelated research programmes in UK, France and Australia, all using laser Doppler anemometry (LDA) as the measurement technique. In the majority of cases, axisymmetric flow is observed for the laminar and turbulent flow conditions, although asymmetry due to the Earth's rotation is evident for the laminar flow of a Newtonian fluid of low viscosity (i.e. low Ekman number). The key point, however, is that for certain fluids, both yield-stress and viscoelastic (all fluids in this study are shear-thinning), asymmetry to varying degrees is apparent at all stages of transition from laminar to turbulent flow, i.e. from the first indications to almost fully developed turbulence. The fact that symmetrical velocity profiles are obtained for both laminar and turbulent flow of all the non-Newtonian fluids in all three laboratories leads to the conclusion that the asymmetry must be a consequence of a fluid-dynamic mechanism, as yet not identified, rather than imperfections in the flow facilities.